A nanocomposite particle of silicon oxide and carbon (SiOx/C) having a core-shell structure and its use as an anode material is known from Jing Wang et al. “Nano-sized SiOx/C composite anode for lithium ion batteries”, Journal of Power Sources 196 (2011) 4811-4815. The SiOx forming the core, presents a structure comprising amorphous Si clusters and ordered SiO2 domains. According to the mentioned publication, the nano feature of the SiOx/C particles combined with a relatively good electronic conductive nature of the carbon coating layer, ensure a good-rate capability of an electrode containing thereof.
It was however observed that the properties of the negative electrode (anode) material containing the above mentioned core-shell particles can be improved. In particular it was observed that for the known anode material, an efficient lithium percolation is difficult to achieve. It was also observed that having an optimum conductivity is important for achieving good properties and that the conductivity of the known anodes can be further optimized. Moreover, the particles forming the known anodes are difficult and expensive to manufacture.
With the development of mobile electronic equipment, transportation, renewable-energy sectors, there is a strong demand for secondary batteries with higher energy density. Compared to other secondary battery systems, lithium-ion batteries (LIBs) have many advantages in terms of high energy and power densities, long cycle life, low self-discharge, high operating voltage, wide temperature window, and no “memory effect”. The state-of-the-art anode material for LIBs is graphite due to its long cycle life, abundant material supply and relatively low cost. However, the graphite anode shows low energy density (372 mAh/g) and safety issues related to lithium deposition under overcharge conditions. Therefore, much attention is paid recently to develop alternative anode materials with enhanced safety, high specific capacity and also long cycle life.
Silicon is of special interest because of its potentially largest theoretical capacity of around 3600 mAh/g for Li15Si4 alloy. However, the application of silicon anodes has been hindered by rapid capacity fading upon charge/discharge cycling. The capacity loss was believed to be mainly due to expansion/contraction of the material during the insertion/extraction of Li+, which leads to a strong mechanical stress of the crystallites and results in the loss of the electrical contact. That eventually results in the rapid loss of reversible capacity upon prolonged cycling. Many strategies have been proposed to improve the cyclability of Si-based anode materials, such as employing nano-technology, alloying with other elements and coating/mixing with carbon.
An alternative material to pure Si is silicon oxide or SiO. So-called “Silicon monoxide” SiO, if it exists, would be the only compound of silicon in which silicon is bivalent. In recent years, experimental evidence taken via various methods has confirmed that silicon(II) oxide does not exist as a distinct phase but as a mixture of Si and SiO2. The interface for this mixing occurs over the scale of 3 to 4 nm. Therefore, as noted by Schnurre et al. in Thermodynamics and phase stability in the Si—O system. J. Non-Cryst. Solids 2004, 336, 1-24, “amorphous SiO is not a classical homogeneous single phase, yet because of this small domain size it is also not a classical heterogeneous two-phase mixture”. It is referred to by many as a random-mixture (RM) model, speculating that over certain small domains, silicon is bonded to only silicon or only oxygen, corresponding to an intimate, two-phase mixture of Si and SiO2. This is confirmed by the 29Si MAS-NMR spectra of SiO showing two different resonances whose chemical shift values are close to those of elemental state Si and SiO2, suggesting that the RM model is the more appropriate description for SiO microstructure.
SiOx material is a potential parent material for Si-based anode material owing to the irreversible generation of Li2O/Li4SiO4 and Si during the first lithiation process. The in-situ formed Si during the first lithiation process should be nano-sized and dispersed uniformly in the simultaneously formed Li2O/Li4SiO4 matrix. The latter as inactive component can prevent the active Si cluster from aggregation, and thus improve the cycling stability of Si-based materials.
Commercial SiOx powder is usually prepared by (1) heating a mixture containing silicon and silicon dioxide in an inert gas atmosphere or in vacuum at a high temperature to generate SiO gas, and feeding oxygen gas to the SiO gas to form a gas mixture, and depositing the gas mixture on a surface of a cooled substrate, where the x value is usually more than 1 (see for example US2010/009261A1); (2) mixing and depositing a gas mixture of SiO and Si gases on a substrate, the starting material to generate SiO gas being a mixture of a silicon oxide powder or a silicon dioxide powder with a metal silicon powder, where the x value is usually less than 1 (see for example US2007/0254102A1). The preparation methods of SiOx by simultaneously generating silicon and silicon oxide vapours suffers from the high working temperature (more than 2000° C.) due to the low vapour pressure of silicon and silicon oxide, which results in high cost and low yields.
With respect to the preparation method of composites of SiOx/graphite, SiOx/carbon, and SiOx/graphite/carbon, commercial SiOx powder is commonly used to ball mill with graphite to form SiOx/graphite composite anode materials, while disordered carbon can be formed on the surface of SiOx particles by CVD, sol-gel, hydrothermal methods, etc., followed by a heat treatment.
An object of the invention is to provide an optimized negative electrode material comprising SiOx and carbon and a method for preparing the negative electrode material SiOx/C for use in LIBs, with simple and flexible synthesis conditions. A further object is to provide a negative electrode material, which exhibits high specific capacity and good cycling stability. A yet further object of the present invention is to provide a negative electrode material characterised by an optimum lithium percolation.